In general, a cathode ray tube (CRT) contains a vacuum envelope, an electron gun for producing an electron beam and, opposite the electron gun, a screen that is coated with a phosphor material that produces light when impacted by the electron beam. The beam is positioned using a magnetic yoke that responds to control signals from a CRT control circuit that raster scans the electron beam across the screen. Furthermore, as is well known in the art, to properly focus the beam on the screen, a typical CRT uses either a magnetic or electrostatic beam focusing apparatus. The focusing apparatus is generally static in nature, in that, optimal focus is achieved at the center of the screen and the beam becomes defocused at the extreme edges of the screen. In low resolution CRTs, the image produced by the defocused beam is not noticeably distorted.
In some high resolution CRTs, the focusing apparatus (lens) is dynamically adjusted such that the focal length of the focusing apparatus is altered as the beam approaches the edges of the screen. Such dynamic focusing is necessary as the ratio of the distance from the electron gun to the screen at the screen center to the distance from the electron gun to the screen at the screen edge becomes smaller. This ratio is referred to herein as the distance ratio. In a typical CRT containing dynamic focusing, the static signal used to focus the beam at the center of the screen is altered to focus the beam at the screen edges. For example, in a CRT using a magnetic focus apparatus, the static focusing current is altered as the beam is scanned on the screen such that focus is maintained over substantially the entire screen. In contrast, in a CRT using an electrostatic focusing element, the static focusing voltage is varied in accordance with the beam position such that the beam remains in focus.
However, using either dynamic electrostatic or dynamic magnetic focusing in a CRT having a small distance ratio requires a great amount of power to drive the focusing apparatus. Also, for certain large, flat screens, it is impossible to achieve a dynamic range sufficient for the focusing apparatus to accurately focus the beam at all locations on the screen.
Therefore, a need exists in the art for apparatus capable of efficiently focusing the electron beam at any location on a CRT screen.